1. Field of the Invention
The present invention relates to a power supply apparatus having a reverse flow-preventive diode provided in an output line thereof, and to a power supply system having a plurality of such power supply apparatuses connected in parallel to each other.
2. Description of the Related Art
There has been proposed a power supply system having a plurality of power supply apparatuses connected in parallel to each other. Since the power supply apparatuses are connected in parallel to each other, the power supply system can supply a load with a large power. And, even though any one of the power supply apparatuses fails, the failed one can be backed up by another normal one.
FIG. 1 shows a conventional power supply system having two flyback type switching converters connected in parallel to each other. The conventional power supply system is generally indicated with a reference 100.
As shown, the conventional power supply system 100 includes a first switching converter 101 and a second switching converter 102, which are connected in parallel to a load 103. The first and second switching converters 101 and 102 are identical in circuit configuration with each other. Therefore, the circuit configuration of only the first switching converter 101 will be explained hereinafter.
The first switching converter 101 includes an AC input terminal 111, an input filter 112 and a rectifying circuit 113.
The first switching converter 101 is supplied with, for example, a commercial AC power by applying a commercial AC voltage to the AC input terminal 111. The AC voltage is then applied to the input filter 112. The input filter 112 is provided to remove a power noise from the input AC voltage, and then the AC voltage with no power noise is applied to the rectifying circuit 113. The rectifying circuit 113 rectifies the AC voltage to provide a DC input voltage (Vin) of a predetermined value.
The first switching converter 101 further includes a transformer 114 having a primary winding 114a and a secondary winding 114b, a switching element 115, a pulse width modulating (PWM) circuit 116, a rectifier diode 117 and a smoothing capacitor 118.
The primary winding 114a of the transformer 114 has one end thereof connected to the rectifying circuit 113 which applies the DC input voltage (Vin) to that end of the primary winding 114a. The primary winding 114a of the transformer 114 has the other end thereof connected to the ground via the switching element 115. The switching element 115 is, for example, a field effect transistor (FET). The switching element 115 has the gate thereof connected to the PWM circuit 116, and is driven in a pulsed manner by a PWM signal supplied from the PWM circuit 116. The switching element 115 is pulse-driven by the PWM signal to switch a current through the primary winding 114a of the transformer 114.
The secondary winding 114b of the transformer 114 has one end thereof connected to the ground. The secondary winding 114b of the transformer 114 has the other end thereof connected to the anode of the rectifier diode 117. The rectifier diode 117 has the cathode thereof connected to the ground via the smoothing capacitor 118. The connection point at which the cathode of the rectifier diode 117 and the smoothing capacitor 118 are connected to each other will be referred to as D point. At the secondary winding 114b of the transformer 114, a voltage is induced from the primary winding 114a due to the switching operation of the switching element 115. The rectifier diode 117 rectifies, and smoothing capacitor 118 smooths, the voltage induced at the secondary winding 114b to generate a DC voltage (VP) at the D point.
The first switching converter 101 further includes a voltage divider 119, a voltage divider 120, a differential amplifier 121 to detect output voltage error, a reference voltage source 122 to generate a reference voltage (Vref) and a photocoupler 123 consisting of a light emitting diode 124 and a phototransistor 125.
The voltage dividers 119 and 120 are connected in series between the D point and ground. The differential amplifier 121 has an inverting input terminal connected to a connection point between the voltage dividers 119 and 120, and has a non-inverting input terminal connected to a positive terminal of the reference voltage source 122. The reference voltage source 122 has a negative terminal connected to the ground. The light emitting diode 124 of the photocoupler 123 has the anode and cathode thereof connected to the D point and the output terminal of the differential amplifier 121, respectively. The phototransistor 125 of the photocoupler 123 has the emitter and collector thereof connected to the ground and PWM circuit 116, respectively.
The differential amplifier 121 is supplied at the inverting input terminal thereof with a DC voltage (VP) produced by dividing the DC voltage (VP) at the D point at a ratio of voltage division between the voltage dividers 119 and 120. Also, the differential amplifier 121 is supplied at the non-inverting input terminal thereof with a reference voltage (Vref) generated by the reference voltage source 122. The differential amplifier 121 amplifies a difference in voltage between the non-inverting and inverting input terminals thereof to provide a difference voltage, namely, an error voltage, between the voltage-divided DC voltage (VP) and reference voltage (Vref). The error voltage is applied to the PWM circuit 116 via the photocoupler 123. The PWM circuit 116 varies, based on the error voltage, the duty ratio of the PWM signal and switches the switching element 115 such that the DC voltage (VP) at the D point is stabilized at a constant level.
The first switching converter 101 further includes a reverse flow-preventive diode 126, an output resistor 127, a positive output terminal 128 and a negative output terminal 129. The reverse flow-preventive diode 126 has the anode thereof connected to the D point and the cathode thereof connected to the positive output terminal 128 via the output resistor 127. The negative output terminal 129 is connected to the ground.
The conventional power supply system 100 has the first and second switching converters 101 and 102 connected in parallel to each other, and supplies the load 103 with a power.
More specifically, the positive output terminal 128 of the first switching converter 101 and the positive output terminal 128 of the second switching converter 102 are connected to each other and to the positive power input terminal 104 of the load 103. Furthermore, the negative output terminal 129 of the first switching converter 101 and the negative output terminal 129 of the second switching converter 102 are connected to each other and to the negative power input terminal 105 of the load 103.
As in the above, the conventional power supply system 100 supplies the load 103 with a power which is larger than that generated by one switching converter.
Generally, in case a plurality of power supply apparatuses are connected in parallel to each other, there takes place a very small difference in output voltage between the power supply apparatuses.
Thus, in the conventional power supply system 100, the reverse flow-preventive diode 126 is provided to prevent a current from flowing from the switching converter which generates a high voltage to the switching converter which generates a low voltage, and the output resistor 127 is provided to absorb the potential difference, to minimize the difference between the currents supplied from the two switching converters 101 and 102, respectively, to the load 103 and to supply a power to the load 103 very efficiently.
It is assumed now that the voltage (VP) generated at the D point of the first switching converter 101 has a value VP1 voltage (VP) generated at the D point of the second switching converter 102 has a value VP2 and that VP1 less than VP2. It is also assumed that a DC current I1 is delivered at the positive output terminal 128 of the first switching converter 101, and a DC current I2 is delivered at the positive output terminal 128 of the second switching converter 102.
In this case, if the reverse flow-preventive diode 126 is not provided in the power supply system 100, a part (reverse flow Ir) of the DC current I2 from the second switching converter 102 flows into the voltage dividers 119 and 120 of the first switching converter 101, thus generating unstable DC voltage (VP1), which is not constant, at the D point. However, since the first switching converter 101 has the reverse flow-preventive diode 126, the reverse flow Ir will not flow into the voltage dividers 119 and 120, thus a constant and stable DC voltage (VP1) is generated at the D point.
Further, if the output resistor 127 is not provided in the power supply system 100, the second switching converter 102 in which DC voltage (VP) at the A point is high, will provide 100% of a load current I0, while the first switching converter 101 in which DC voltage (VP) at the A point is low, will provide no load current I0. In the power supply system 100, however, as the DC currents I1 and I2 output from the positive output terminals 128, respectively, increase, a voltage (VR) generated across the output resistor 127 increases, while an output voltage (VS) generated at the positive output terminal 128 drops linearly. Accordingly, both the first switching converter 101 and second switching converter 102 in the power supply system 100 will evenly contribute themselves to supply of the load current I0.
FIG. 2 shows a relationship between the output currents I1, I2 from the first switching converters 101 and 102, and the output voltage (VS) supplied from the power supply system 100 to the load 103.
As shown in FIG. 2, even if there is generated a very small difference between the voltage VP1 at the D point of the first switching converter 101 and the voltage VP2 at the D point of the second switching converter 102, the output resistor 127 causes a linear voltage drop (VR) since the output resistor 127 is provided between the D point and the positive output terminal 128. Thus, also when the output voltage (VS) applied from the positive output terminal 128 to the load 103 is constant, a current for supply to the load 103 is supplied from each of the first switching converter 101 and second switching converter 102. Specifically, when the output voltage (VS) is, for example, 12V, the first switching converter 101 will provide an output current of 4A from the positive output terminal 128 thereof, while the second switching converter 102 will provide an output current of 6A from the positive output terminal 128 thereof. In case the resistance value of the output resistor 127 is larger, the ratio of the voltage drop caused by the output resistor 127 becomes large, while the difference between output currents provided by the first switching converter 101 and second switching converter 102 are reduced, as shown FIG. 3.
As in the above, there is provided a reverse flow-preventive diode 126 in either of the first and second switching converters 101 and 102. Like the output resistor 127, the reverse flow-preventive diode 126 has such a nature that when the current through the reverse flow-preventive diode 126 has a larger value than predetermined, a drop voltage (VF) increases in proportion to the flowing current. Thus, when the output current value is larger than predetermined, the reverse flow-preventive diode 126 can drop the output voltage (VS) at the positive output terminal 128 linearly similarly to the output resistor 127.
The drop Vdp of the output voltage (VS) supplied from the positive output terminal 128 of each of the first and second switching converters 101 and 102 will be as follows in case the reverse flow-preventive diode 126 and output resistor 127 are provided in each switching converter.
xe2x80x83Vdp=VF+VR
When the current through the reverse flow-preventive diode 126 has a smaller value than predetermined, the ratio of the drop voltage (VF) will be large, and the drop voltage (VF) does not increase in proportion to the flowing current. Specifically, FIG. 4 shows the volt-ampere characteristics of a Schottky diode. When the current through the Schottky diode is smaller than 2A, the voltage varies significantly larger than the current, as shown in FIG. 4.
Furthermore, the temperature characteristics of the reverse flow-preventive diode 126 of the first switching converter 101 and that of the second switching converter 102 are not identical with each other, or have some errors. Also, the output characteristics of the reverse flow-preventive diode 126 is affected by the environmental temperature and chronological change. Similarly, the temperature characteristics of the output resistor 127 of the first switching converter 101 and that of the second switching converter 102 are not identical with each other, or have some errors.
Thus, in the power supply system 100, since the voltage drop of each of the power supply apparatuses is not linear, there is cause a large difference between the DC current I1 from the first switching converter 101 and DC current I2 from the second switching converter 102, and thus one of the switching converters 101 and 102 will be more contributed to providing the load current I0 than the other. This one-sided contribution to providing the load current I0 will adversely affect the product reliability.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks by providing a power supply apparatus which is not affected by a voltage fluctuation of a reverse flow-preventive diode provided in an output line thereof, and can provide a stable output voltage controlled with high accuracy, and a power supply system having a plurality of such power supply apparatuses connected in parallel to each other.
According to the present invention, there is provided a power supply apparatus including:
a DC voltage source;
an output terminal to supply a power to an external load, the external load being connected to the output terminal;
a diode provided between the DC voltage source and the output terminal, the diode having the anode thereof connected to the DC voltage source and having the cathode thereof connected to the output terminal;
means for detecting a forward voltage of the diode;
means for detecting a forward current of the diode; and
means for controlling a DC voltage generated from the DC voltage source;
the control means controlling a DC voltage generated from the DC voltage source so that the anode potential of the diode remains constant, and dropping the output terminal voltage in accordance with the forward current detected by the forward current detection means, and raising the anode potential of the diode in accordance with the forward voltage detected by the forward voltage detection means.
In the power supply apparatus, the diode provided between the DC voltage source and the output terminal works as a reverse flow-preventive diode. And a DC voltage generated from the DC voltage source is controlled so that the anode potential of the diode remains constant, and the output terminal voltage is dropped in accordance with the forward current detected by the forward current detection means, and the anode potential of the diode is raised in accordance with the forward voltage detected by the forward voltage detection means.
According to the present invention, there is also provided a power supply system having a plurality of power supply apparatuses connected in parallel to an external load, each of the power supply apparatuses including:
a DC voltage source;
an output terminal to supply a power to an external load, the external load being connected to the output terminal;
a diode provided between the DC voltage source and the output terminal, the diode having the anode thereof connected to the DC voltage source and having the cathode thereof connected to the output terminal;
means for detecting a forward voltage of the diode;
means for detecting a forward current of the diode; and
means for controlling a DC voltage generated from the DC voltage source;
the control means controlling a DC voltage generated from the DC voltage source so that the anode potential of the diode remains constant, and dropping the output terminal voltage in accordance with the forward current detected by the forward current detection means, and raising the anode potential of the diode in accordance with the forward voltage detected by the forward voltage detection means.
In the power supply system, the diode provided between the DC voltage source and the output terminal of each of the power supply apparatuses works as a reverse flow-preventive diode. And a DC voltage generated from the DC voltage source is controlled so that the anode potential of the diode remains constant, and the output terminal voltage is dropped in accordance with the forward current detected by the forward current detection means, and the anode potential of the diode is raised in accordance with the forward voltage detected by the forward voltage detection means.